As a method for purifying fermentation alcohol liquid discharged from a fermenter, a conventional method is separating and concentrating an alcohol-water mixture from the fermentation alcohol liquid (mash) by a mash column; further concentrating the alcohol-water mixture substantially to an azeotropic composition by a concentrating column; and then treating it by an azeotropic distillation column to give high-purity anhydrous alcohol.
Patent Reference No. 1 has disclosed a method of distilling an azeotropic mixture in a distillation column, feeding a vapor of the azeotropic mixture to a membrane separator and separating it through a separation membrane.
Patent Reference No. 2 has proposed a separation apparatus consisting of a distilling means and a membrane separating means equipped with a zeolite membrane which separates a mixed vapor distilled from the top of the distillation means. It has been described therein that an ethanol/water mixture is converted to a vapor having an ethanol concentration of 91.0% by weight by the distilling means, and then, purified by the zeolite membrane to an ethanol concentration of 99.5% by weight.